1. Field of the Invention
The present invention relates to an organic light emitting display.
2. Discussion of the Background
An organic light emitting display (OLED) is a self emissive display device, which displays images by exciting an emissive organic material to emit light. The OLED includes an anode (hole injection electrode), a cathode (electron injection electrode), and an organic light emission layer interposed there between. When the holes and the electrons are injected into the light emission layer, they recombine to form exitons, which emit light when they transition from an excited state to a ground state. The light emission layer may further include an electron transport layer (ETL) and a hole transport layer (HTL) as well as an electron injecting layer (EIL) and a hole injecting layer (HIL) for enhancing the light emission. Each pixel of the OLED includes two thin film transistors (TFTs), i.e., a switching TFT and a driving TFT. The current for light emission is driven by the driving TFT and the amount of the current driven by the driving TFT is controlled by the data signals from the switching TFT.
A plurality of pixels of the OLED, each including an anode, a cathode, and a light emission layer, are arranged in a matrix and driven in passive matrix (or simple matrix) addressing or active matrix addressing.
The passive matrix type OLED includes a plurality of anode lines, a plurality of cathode lines intersecting the anode lines, and a plurality of pixels, each including a light emission layer. The selection of one of the anode lines and one of the cathode lines cause light emission of a pixel located at the intersection of the selected signal lines.
The active matrix type OLED includes a plurality of pixels, each including a switching transistor, a driving transistor, and a storage capacitor, as well as an anode, a cathode, and a light emission layer. The OLED further includes a plurality of gate lines transmitting gate signals and a plurality of data lines transmitting data voltages. The switching transistor is connected to one of the gate lines and one of the data lines and transmits the data voltage from the data line in response to the gate signal. The driving transistor receives the data voltage from the switching transistor and drives a current having a magnitude determined depending on the difference between the data voltage and a predetermined voltage, such as a supply voltage. The current from the driving transistor enters the light emission layer to cause light emission having an intensity depending on the current. The storage capacitor is connected between the data voltage and the supply voltage to maintain their voltage difference. The gray scaling of the active matrix type OLED is accomplished by controlling the data voltages to adjust the current driven by the driving transistor. The color representation of the OLED is obtained by providing red, green and blue light emission layers.
The light emission of the organic light emission layer is generated in a region having a thickness of several nanometers to dozens of nanometers. Since the luminance is proportional to the area of the light emission layer, it is important to secure a high aperture ratio for increasing the luminance level.
However, even after obtaining a high aperture ratio, increasing the luminance level is limited due to the reflection of light at the surface of the light emission layer, which escapes through lateral surfaces.